Coloured scanning electron micrograph (SEM) of Purkinje fibres running over heart muscle fibres. Purkinje fibres are modified cardiac muscle fibres which originate from the atrioventricular node and spread into the two ventricles. They transmit the electrical impulse from the atrioventricular node to the ventricles enabling their almost simultaneous contraction. The spread of excitation through the ventricles from the atrioventricular node is extremely rapid, moving at one to four meters per second. Magnification: x1,200 when shortest axis printed at 25 millimetres.